1. Field of the Invention
The present invention relates to a metal base substrate which includes a semiconductor element and other elements mounted thereon and also provides a heat release function, and a method for manufacturing the metal base substrate, and more particularly, the present invention relates to a metal base substrate which has a metal substrate combined with a ceramic layer configured with the use of a low-temperature sintering ceramic material, and a method for manufacturing the metal base substrate.
2. Description of the Related Art
Metal base substrates have a relatively high heat release function, and have been used advantageously for mounting electronic components which require heat release, such as, for example, a semiconductor element. In the case of these metal base substrates, ceramic materials are used as a material constituting a substrate layer combined with a metal substrate.
For example, a low-temperature sintering ceramic material is a ceramic material which can be sintered at a temperature of 1050° C. or less. Therefore, as long as a metal base substrate has a ceramic layer configured with the use of the low-temperature sintering ceramic material and formed on a metal substrate, the raw low-temperature sintering ceramic layer and the metal substrate can be subjected to co-firing without using a metal substrate composed of a metal which has a very high melting point.
However, in the case of the metal base substrate as described above, it is important to keep the low-temperature sintering ceramic layer and the metal substrate from peeling during firing. Therefore, a joining glass layer is formed between the low-temperature sintering ceramic layer and the metal substrate. For example, Japanese Patent Unexamined Application Publication No. 5-270934, Japanese Translation of PCT International Application Publication No. 11-514627, and Japanese Translation of PCT International Application Publication No. 11-511719 disclose inventions which relate to a metal base substrate which is obtained by forming a joining glass layer on a metal substrate, and forming a low-temperature sintering ceramic layer thereon, and applying co-firing.
For the configuration according to the prior art, there is a need to form the joining glass layer in advance between the metal substrate and the low-temperature sintering ceramic layer. In the case of forming the joining glass layer in advance as just described, the joining glass layer has to be formed thickly in order to bind the metal substrate and the low-temperature sintering ceramic layer, and for example, Japanese Patent Unexamined Application Publication No. 5-270934 discloses the joining glass layer formed with a thickness of approximately 25 μm, whereas Japanese Translation of PCT International Application Publication No. 11-514627 discloses the joining glass layer formed with a thickness of approximately 35 μm. It is to be noted that Japanese Translation of PCT International Application Publication No. 11-511719 fails to disclose any specific thickness.
On the other hand, this joining glass layer is intended to function as a joining material, and thus a very weak layer in terms of flexure strength.
Moreover, when the joining glass layer is thick as described previously, flexure makes cracking more and more likely to be caused. This is because when the joining glass layer undergoes a deflection, the difference is further increased between stress caused on one principal surface side of the joining glass layer and stress caused on the other principal surface side thereof as the joining glass layer is thicker.
In addition, the prior art requires two firing steps including a firing step for forming the joining glass layer on the metal substrate; and a firing step after forming the low-temperature sintering ceramic green layer thereon, thereby resulting in a complicated process, and also requires a lot of energy for heating.